1. Field of the Invention
The present invention relates to a magnetoresistive effect memory using a magnetoresistive effect element.
2. Description of the Related Art
Various types of magnetoresistive effect memories have heretofore been proposed. Recently, a magnetoresistive effect memory using a ferromagnetic tunnel junction element that shows a tunneling magnetoresistive (TMR) effect, such as a magnetoresistive random access memory (MRAM), has been attracting attention. As a TMR effect element, a magnetic tunnel junction (MTJ) element that uses the change of magnetic resistance caused by a spin-polarized tunneling effect is generally used. The MTJ element assumes a low resistance state or a high resistance state in accordance with the relative magnetization arrangement of two ferromagnetic layers, and stores data by matching “0” or “1” to this resistance state.
One method of writing data in the magnetoresistive effect memory is a spin transfer writing method that uses the spin torque of a current (electrons) to change the direction of the magnetization of a magnetic layer. In the spin transfer writing method, a write current equal to or more than a certain current value is passed through the MTJ element. Then, the relative magnetization arrangement of two ferromagnetic layers constituting the MTJ element is changed from a parallel state to an antiparallel state or from an antiparallel state to a parallel state in accordance with the direction in which the write current runs through the MTJ element, whereby data is written. When data is read from the magnetoresistive effect memory, the data is identified by using a resistance difference between the high resistance state and low resistance state of the MTJ element. In reading data, a current (read current) is directly passed through the MTJ element as in the case of data writing.
For example, as described in Z. Li and S. Zhang, Physical Review B, Vol. 69, 134416 (2004) (hereinafter referred to as Reference 1), it has been considered that the probability of magnetization inversion of the MTJ element attributed to spin transfer can be indicated by a simple thermally-activated process which is represented by (Expression 1):
                              P          ⁡                      (            τ            )                          =                  1          -                      exp            ⁡                          [                                                -                  τ                                ⁢                                                                  ⁢                f                ⁢                                                                  ⁢                exp                ⁢                                  {                                                            -                                                                        Δ                          ⁢                                                                                                          ⁢                                                      E                            a                                                                                                                                k                            B                                                    ⁢                          T                                                                                      ⁢                                          (                                              1                        -                                                  I                                                      I                                                          c                              ⁢                                                                                                                          ⁢                              0                                                                                                                          )                                                        }                                            ]                                                          (                  Expression          ⁢                                          ⁢          1                )            
In the magnetization inversion model represented by (Expression 1), the probability (1-P(τ)) that magnetization is not inverted by the spin transfer exponentially decreases with respect to time τ of current supply to the MTJ element. Moreover, in the model represented by (Expression 1), magnetization is inverted by the spin transfer with a finite probability immediately after the supply of a pulse current, that is, even in an extreme case where the pulse width τ of a current is directed to 0.
On the other hand, according to H. Tomita et al., Applied Physics Express, Vol. 1 (2008) 061303 (hereinafter referred to as Reference 2), there is a period of time in which a probability of the magnetization inversion is nearly zero immediately after the supply of a current to the MTJ element is started, and after this given length of time, the probability of the magnetization inversion increases, as represented by (Expression 2) and (Expression 3):P(t)=0(t<t0)  (Expression 2)P(t)=1−exp(t−t0)/τ](t≧t0)  (Expression 3)
In a spin-transfer-type MRAM, the value of a write current is set at a current value higher than a threshold (hereinafter referred to as an inversion threshold) at which magnetization is inverted by spin transfer. The value of a read current is set at a current value lower than the inversion threshold.
However, the inversion threshold also varies due to characteristic variations of MTJ elements constituting a memory cell array. Moreover, when data is repeatedly written into the same element, there is a phenomenon in which the inversion threshold of this element fluctuates.
Therefore, when the value of the read current is set close to the average of the inversion thresholds in the memory cell array, the magnetization of the MTJ element having a low inversion threshold is erroneously inverted by the read current, and read disturb may occur.
In order to prevent the occurrence of the read disturb, the value of a read current has to be much lower than the inversion threshold. However, if the value of the read current is lower, a read signal (the potential variation of a bit line) is smaller, and sufficient reading sensitivity is not obtained.